This invention relates to an improved procedure for testing optical fiber for bandwidth performance and, more particularly, the bandwidth of a multimode fiber.
Presently, there is a great deal of interest in optical local area networks (LANs) operating at speeds of one gigabit per second (Gb/s) or more. With a gigabit Ethernet standard now in place, this trend is expected to accelerate. In order to achieve high bit rates, these systems require lasers to launch the optical signals, with the leading candidates being vertical cavity surface emitting lasers (VCSELs) and Fabry-Perot (FP) lasers to keep costs down. Additionally, the fiber of choice is multimode fiber, both due to its ease of installation compared to singlemode fiber, and to retain the use of a large installed base of multimode fiber.
The transmission characteristics of a fiber are very much dependent upon the fiber""s index of refraction configuration, and especially on its variation in the radial direction, An optical fiber may transmit light in multiple modes (electromagnetic configurations), or in only a single mode, depending upon the fiber""s core size and index of refraction as well as the launch angle and wavelength of the light being transmitted. Multimode fibers typically have a radially graded index of refraction, although certain step-indexed radial configurations are suitable for the transmission of multiple modes. The radial gradation is used to minimize the xe2x80x9cmode dispersionxe2x80x9d pulse-broadening effect, which is due to different transmission paths associated with the each of the modes. For example, in step-index multimode fibers, lower-order modes are transmitted essentially down the center of the fiber, while higher-order modes are transmitted down the fiber along paths that oscillate back and forth from the center of the fiber core to its periphery. The longer optical path lengths associated with higher order modes generally result in longer transit times. A given pulse transmitted through the fiber is transmitted as a combination of many possible modes. Higher order modes arrive later than the lower order modes because they traverse longer path lengths. Consequently the width of the pulse is significantly broadened (i.e., spread out in time) and the bandwidth is, consequently, decreased. However, in a graded index multimode fiber, the index of refraction decreases at larger radii, and this results in increased velocity for the higher order modes that spend more of their time at the periphery of the fiber core. This increase in velocity tends to compensate for the longer path lengths of higher modes, and approximately equalizes the transit times associated with the various modes thereby minimizing the mode-dispersion effect. Therefore, the bandwidth of the multimode fiber is increased.
For the purpose of standardized measurement, a modal power distribution has been chosen that equally excites all modes in a fiber. This is known as the xe2x80x9coverfilled launchxe2x80x9d (OFL), and various standards bodies specify it""s use in characterizing the fiber bandwidth for purposes of trade and commerce. (Historically, multimode fibers were used with light-emitting diodes (LEDs), which very nearly launch equal power into every mode.) It is expected that optical LANs of the future will employ multimode fiber with laser launches, which are far from overfilled. And while OFL has been successful at predicting the performance of fibers when used with LED sources, conventional bandwidth predictions using OFL are nearly useless for predicting the performance of optical fibers that are coupled to laser sources in high-bit-rate systems. Laser sources excite only some of the modes, and the resulting bandwidth of the fiber depends strongly on which modes are excited by a particular source, The effective bandwidth of a fiber, when used with a particular source, can be much higher or lower than that obtained with the OFL. In order to develop fast, reliable, low-cost systems, it is crucial to understand the behavior and predict the bandwidth of multimode fiber under restricted launch conditions. Accordingly, what is needed is a method of qualifying a multimode optical fiber for bandwidth performance when used with a laser source.
The present invention combines a known modal power distribution (MPD) excited by a laser source of optical power with the differential mode delay (DMD) characteristic of a multimode optical fiber in order to determine its effective bandwidth when used with the laser source. The xe2x80x9cmode dispersionxe2x80x9d is characterized by measuring the DMD of the fiber using light pulses that excite only a small number of the fiber""s propagation modes. Light pulses are injected into one end of the fiber at various radial distances from its center, and differential delay data is obtained at the other end of the fiber. This data is combined with the MPD excited by the laser source to determine an impulse response. Standard methods are then used to transform the impulse response into the frequency domain to determine the power spectrum and, hence, the bandwidth of the multimode fiber. Advantageously, this technique for measuring the bandwidth of a multimode fiber predicts its performance more accurately than traditional OFL measurements.
In a first embodiment of the invention, the raw DMD data (time-domain traces) are individually weighted by the MPD excited by the laser source and superposed to determine the impulse response of the multimode fiber.
In a second embodiment of the invention, the delay times of the individual mode groups are extracted from the measured DMD data. The delay times are then deconvolved using a particular algorithm by a method in which the fiber modal delay times for the individual mode groups from the DMD data are obtained using a deconvolution algorithm. A Gaussian impulse response is assumed for each mode group. The group velocities, attenuation and intra-mode-group dispersion of each mode group of the fiber are determined by applying a deconvolution algorithm to the DMD data. These modal impulse responses can the be superposed, weighted by the MPD excited by the source,:to predict the impulse response of the fiber. Standard methods are then used to determine the bandwidth of the multimode fiber from the impulse response (i.e., map the time-domain response into the frequency domain).
In a preferred embodiment of the invention, the DMD measurement consists of scanning a singlemode fiber across the input end of the test fiber and launching a short pulse at each position. The total power at the output end of the fiber as a function of time is measured and stored.
In a preferred embodiment of the invention, a reverse DMD measurement is made to characterize a laser source. The reverse DMD measurement consists of launching short impulses from the source under study into a test fiber. A singlemode fiber is scanned across the output end of the test fiber and, at each position, power exiting the singlemode fiber is stored as a function of time.